That greatly depends on the type of applications you run and the toolkits and font rendering they use. My most commonly used applications (terminal emulator, Emacs) do their font rendering using the the glyph compositing functionality of the RENDER extension. Server side glyphs are created when a font is loaded and all the compositing is done on the server side based on the client's CompositeGlyphs requests. Same for images (in Emacs) using CreatePixmap and CopyArea.